July 8, 2021
SACRAMENTO - The California Geological Survey (CGS) has released new interactive Tsunami Hazard Area maps for Santa Barbara and San Luis Obispo counties, useful for evacuation planning in an extraordinary event. These maps allow users to type in an address to determine whether the property is within a tsunami hazard area. Local tsunami evacuation material will be added to the online map interface as it is developed.
Working with the California Governor's Office of Emergency Services (Cal OES), these Tsunami Hazard Area maps include new data and improved computer modeling results, and replace maps published by CGS in 2009. The new maps are not only based on how far inland a surge of seawater might go in a worst-case scenario but also include minor inland "buffers" to roads and landmarks to clarify where people must evacuate to be safe. These buffer areas also account for potential errors and uncertainties in the modeling.
The maps incorporate hard lessons learned by the nation of Japan a decade ago during the Tohoku-Oki earthquake and tsunami, noted Dr. Steve Bohlen, the Acting State Geologist of California and head of CGS.
“Japan utilized data from several hundred years of tsunami records in its planning, which seemed perfectly reasonable,” he said. “Then it was impacted by a once-a-millennium tsunami. So, we’re taking a very conservative approach and using a thousand-year scenario as the baseline for our new maps, hoping to avoid the tragic loss of life experienced in Japan.
“While damaging tsunamis are infrequent in California, they have and do happen. If you live on or visit the coast, you need to be aware of this potential hazard.”
While the new maps for the two counties take a variety of potential tsunami sources into account, the worst-case scenario was modeled from a magnitude 9.3 earthquake in the eastern Aleutian Islands. Examples of new information provided by these maps:
- The Santa Barbara Airport area saw both modest increases and decreases in the tsunami hazard. Most areas that saw in increase in hazard are low-lying, undeveloped areas such as canals or wetlands. The east-west airport runway saw a decrease in tsunami hazard.
- The tsunami hazard moderately increased in the Santa Barbara and Carpinteria areas based on an improved understanding of tsunami sources and new modeling results.
- For most other parts of the county, the tsunami hazard remains the same.
- The biggest increases in the tsunami hazard are in Cayucos, where the highest run-up elevation went from about 30 feet to about 50 feet in a few locations. The entire area east of the pier and on the ocean side of South Ocean Avenue may be exposed to hazards in extreme events.
- There are modest tsunami-hazard increases in Los Osos, San Simeon, and the north part of Morro Bay near Highway 1.
- For the southern part of the county, the tsunami hazard either remains the same or – in Avila Beach and Oceano – is slightly reduced.
According to the large Aleutian Island scenario used by CGS, the first surges would reach the counties in five to six hours.
“That time will go by fast, as scientists and emergency planners at the federal, state, and local levels will all need to assess the threat level before issuing an evacuation warning,” said Rick Wilson, head of the CGS Tsunami Program.
"Bottom line: If you’re near the coast and feel strong shaking from a local earthquake or get an official notification to evacuate, move inland or to a higher elevation as soon as possible. A large tsunami surge might be fascinating to watch but you don’t want to be anywhere near it.”
These maps inform the State’s Hazard Mitigation Plan and emergency planning efforts so that state agencies, local governments, tribal governments and others can effectively mitigate and respond to emergencies that might occur in their areas of responsibility.
“We appreciate the collaboration with the Department of Conservation’s California Geological Survey to provide Californians who live, work or visit in a Tsunami Hazard Area the tools they need to develop evacuation plans for themselves, their families and their businesses,” said Lori Nezhura, Deputy Director of Planning, Preparedness and Prevention, Cal OES.
More than 150 tsunamis have hit California’s shore since 1800. Most were barely noticeable, but a few have caused fatalities or significant damage. The 2011 tsunami that devastated Japan also caused $100 million of damage to California ports and harbors, including dangerous conditions and minor to moderate damage to a number of harbors in southern California.
The most destructive tsunami to hit California occurred March 28, 1964. Several surges reaching 21 feet high swept into Crescent City four hours after a magnitude 9.2 earthquake in Alaska, killing 12 and leveling much of the town's business district.
CGS, part of the California Department of Conservation, has issued Tsunami Hazard Area Maps in 13 counties and hopes to update the maps for the remaining seven coastal counties within the next year.
“Hazards management – for geologic events such as tsunamis as well as mining and petroleum production – is one of our department’s pillars, or cornerstones,” Bohlen said.